High tensile strength and steel and method for manufacturing same

ABSTRACT

Method of manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that when Ti added Al killed steel, which contains 0.04 to 0.35% of Ti, under the condition of Ti/C&lt;4, and 0.005, to 0.15% of sol Al, is finishing its hot rolling in a temperature range of 815* to 880*C, at least more than one time of large reduction of more than 28% of draft percentage for one pass are given in a temperature range of 980* to 1100*C.

United States Patent Takechi et a1.

HIGH TENSILE STRENGTH AND STEEL AND METHOD FOR MANUFACTURING SAME Inventors: Hiroshi Takechi; Tsuyoshi Kawano, both of Kisarazu; Toshio Hashizume; Kazuo Koyama, both of Kimitsu; Hiroaki Masui, Chiba, all

of Japan Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Dec. 28, 1973 Appl. No.: 429,274

Foreign Application Priority Data Dec. 31, 1972 Japan 48-2832 Dec. 31, 1972 Japan 48-2833 US. Cl. 148/12 F Int. Cl. C211) 7/14 Field of Search 148/12 F, 12 C Primary ExaminerW. Stallard Alt0rney, Agent, or Firm-Toren, McGeady and Stanger ABS'IRACT Method of manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that when Ti added A1 killed steel, which contains 0.04 to 0.35% of Ti, under the condition of Ti/C 4, and 0.005, to 0.15% of sol A1, is finishing its hot rolling in a temperature range of 815 to 880C, at least more than one time of large reduction of more than 28% of draft percentage for one pass are given in a temperature range of 980 to 1 100C.

9 Claims, No Drawings HIGH TENSILE STRENGTH AND STEEL AND METHOD FOR MANUFACTURING SAME DETAILED EXPLANATION OF THE INVENTION This invention relates to a method for manufacturing high tensile steel, having more than 60 Kg/mm of tensile strength as rolled and at the same time having superior cold workability and excellent low temperature toughness, preferably from a hot rolled steel plate having more than 4.5 mm of thickness.

Important characteristics required for high tensile steel having more than 60 Kg/mm' grade, tensile strength toughness, weldability, cold workability and the like are, and the usual high tensile steel plate may be classified into two types. One type is where the toughness and weldability are regarded as important, since they are mainly used for shipbuilding steel and structural steel. In this case, a thermal refined steel is used which is relatively thick and is mainly subjected to quenching and tempering and the like. Its most important characteristics are toughness and weldability, and it possesses little cold workability propertiesThe other type is steel plate having a comparatively small thickness (for instance, less than 9 mm) which is used for truck frames and the like. In case, its most important characteristic is superior cold workability which is bearable under severe cold working, and particularly under bending working and hole expanding working, while toughness and the like are not as important.

But, recently high tensile steel having a strength of 60-80 Kg/mm grade has come to 'be used even for steel plate for cold working.

In general, the cold workability, toughness and weldability of steel will decrease as its strength increases, so that even high tensile steel for cold working cannot ignore the low temperature toughness of a portion of the plastic deformation in deference to the cold workability. Thus, high teiisile steel having inferior low temperature toughness is in danger of brittle fracture during its use, particularly in cold districts, even though its cold workability is excellent. Therefore, in high tensile steel having a thickness of more than 4.5 mm, it is required to have superior low temperature toughness in addition to cold workability.

Because of this, attempts to develop a high tensile steel, which is superior, not onlyin cold workability, but also in low temperature toughness and weldability. However, the high tensile steel that satisfied all of these required characteristics was very limited and expensive.

Usually, as a high tensile steel for cold working, niobium added steel and vanadium added steel have been widely used. These steel are superior in toughness, however, they have such disadvantages that the strength of niobium steel is limited up to 60 Kg/mm grade and vanadium steel may be produced up to 70 Kg/mm grade but is expensive.

Recently, instead of niobium or vanadium steel, titanium added high tensile steel has been investigated. The titanium added steel has the advantage of being cheaper than niobium steel or vanadium steel and is superior in cold workability. But on the other hand, it is inferior to such steels in toughness which is an essential characteristics of a high tensile steel of more than 4.5 mm of thickness.

ability, by means of manufacturing a titanium added steel having an appropriate composition range under specified hot rolling conditions.

The essential points of the method according to the invention may be enumerated as follows.

1. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that when Ti added Al killed steel, which contains 0.04 to 0.35% of Ti, under the condition of Ti/C 4, and 0.005 to 0.15% of So] Al, is finishing its hot rolling in a temperature range of 815 to 880C, at least more than one time of large reduction of more than 28% of draft percentage for one pass are given in a temperature range of 980 to ll00C.

2. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness characterized in that when Ti added Al killed steel, which contains 0.04 to 0.35% of Ti, under the condition of Ti/C 4, and 0.005 to 0.15% of S01 Al, is finishing its hot rolling in a temperature range of 730 to 815C, total draft percentage below 980C is made more than 50%.

3. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the item (2), characterized in that at least more than one time of large reduction of more than 28% of draft percentage for one pass are given in a temperature range of 980 to ll00C.

4. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the items (1) to (3), characterized in that the Ti added Al killed steel contains 0.03 to 0.20% of C, less than or equal to 0.8% of Si, 0.90 to 2.5% of Mn, less than or equal to 0.020% of S, less than or equal to 0.008% of N, the rest of Fe and inevitable elements.

5. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the item (4), characterized in that it contains one or more than two kinds of Zr 0.01 to 0.15%, La+Ce=0.005 to 0.10%, Nb =0.0l to 0.10%, V=0.0l to 0.10%, B 0.0030%, Ni 0.5%, Cr 0.5%, Cu 0.5%.

6. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the items (1) to (5), characterized in that Ti added Al killed steel is heated above l200C in a heating furnace for slab and hot-rolled subject to predetermined conditions.

7. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the items (1) to (6), characterized in that it is coiled in a temperature range of 500 to 680C, after hot-rolling thereof is finished.

8. Method for manufacturing of high tensile steel having superior cold workability and excellent toughness according to the item (7), characterized in that after its hot-rolling is finished, its cooling velocity is made 3 to 40C/sec until it is coiled in the temperature range of 500 to 680C.

In the invention, the reasons for limitation of preferred chemical compositions and rolling'conditions are described as follows:

C is necessary for obtaining a strength of more than 60 Kg/mm and this object cannot be attained with less than 0.03% C, while more than 0.20% C will result in inferior cold workability, low temperature toughness and weldability. The preferable range for the carbon is 0.05 to 0.15% C.

Ti is a very important element for the invention, because it will remarkably increase the strength of the steel by the precipitation of fine Ti(CN), Ti(CN), and at the same time decrease the plasticity of the non-metallic inclusion and increase the cold workability and impact absorbed energy in the transverse direction. But, the object of the invention cannot be attained by less than 0.04% Ti, while its effect will be saturated with more than 0.35% Ti. It is preferable to add 0.05 to 0.30% Ti in order to realize the characteristics of the invention. Also, it is necessary to make Ti/C 4 to increase the strength efficiently, and when the ratio becomes more than 4, the strength of the steel will decrease.

Si is effective to increase the strength, but when it becomes more than 0.8%, the toughness and weldability will become inferior, so that its upper limit is made as 0.8%. However, if Si is not necessary for the strength, it may not be added. However, normally it is preferable to add more than 0.10% Si in order to obtain the desired strength.

Mn is an essential element to make the steel strong and tough. In the invention, less than 0.9% Mn is effective, and the cold workability is decreased with more than 2.5% Mn, so that the upper limit is 2.5%, but it is preferable to add less than 2.0% Mn in order to minimize the appearance of the upper bainite structure, which is harmful to the toughness.

Al is necessary with 0.005 to 0.15% thereof being present as acid soluble Al to assure the effectiveness of to Ti.

The preferable range is 0.01 to 0.10% sol Al.

S and N are deleterious elements in the steel of the invention, because they will combine with Ti and decrease the amount of effective Ti which will act on the reinforcement of the steel.

Reduction of S and N is extremely effective to toughen the steel, and it is necessary to make the S less than 0.020% and the N less than 0.0080%, respectively, to stabilize the characteristics of the invention.

Advantageously, it is preferred that S and N contents are as small as possible, i.e., S is less than 0.010% and N is less than 0.0060%. Additionally, one or more of the undermentioned elements may be added as occasion demands for the purposes indicated.

Zr will fix the N and S in the steel to increase the effectiveness of the Ti and to improve the characteristics of the invention, so that it is a very important element. However, less than 0.01% Zr is not effective and its effect will be saturated with more than 0.15% Zr. The optimum content of Zr is 0.03 to 0.06%.

Mischmetal will also fix the S in the steel and improve the effectiveness of the Ti as well as the cold workability and toughness. When the mischmetal is present, no effect is observed if (La Ce) is less than 0.005%, and the effect is saturated if (La Ce) is more than 0.10%. The optimum content thereof is (La Ce) 0.005-0.05%.

Nb and V will toughen steel by way of grain refining and stabilize the effect of the invention, and also V will fix N which is deleterious to the invention, so that both of these are effective elements in the steel of the invention. But neither has any effect in the range of less than 0.01%, and in the range of more than 0.10%, not only is the effect is, but the steel becomes very expensive. The optimum content thereof is 0.01 to 0.05%, respectively.

Ni, Cr and Cu will lower the transformation temperature (Ar of steel and are effective for toughening the steel as well as improving the corrosion resistance thereof, so that less than 0.5% of each may be added if needed.

B will fix N which is deleterious to the invention and increase the strength of steel, so that less than 0,0030% thereof may be added if needed.

The reasons for limiting the rolling conditions are as follows.

In the first place, as to the heating temperature for the slab, in order to obtain high strength, it is necessary to elevate the heating temperature so as to maximize the solid solution of Ti in the austenite structure, and to assure a tensile strength of more than 60 Kglmm which being the object of the invention. Particularly, it is necessary to make the heating temperature more than 1200C, and, it is preferable to make the temperature more than l260C for Kg/mm grade and more than 1280 C for Kg/mm grade.

In the next place, the rolling conditions are particularly important in the invention. Namely, the heating temperature for slab is made high to obtain the highest strength in the invention, but under the condition of ordinary hot rolling, the low temperature toughness of steel is remarkably lowered. This is because the austenite grains become remarkably coarse, and in titanium contained steel as well as niobium steel, the low temperature toughness will decrease if the heating temperature is raised to improve the strength. Conversely, if low temperature heating is effected to improve the low temperature toughness, the strength of steel will be lowered. Moreover, the titanium containing steel shows a more remarkable deterioration in toughness from high temperature heating compared with the niobium containing steel.

The inventors succeeded in developing of high tensile steel plate which has high strength, superior toughness and cold workability at the same time by specifying the rolling condition, as the result of various researches about methods for improving toughness for titanium containing steel which is heated at a temperature of more than 1200C.

In the invention, the object is attained by two kinds of rolling conditions, i.e., the high temperature finishing type and low temperature finishing type.

In the first place, the rolling condition of the high temperature finishing type will be explained. Namely, as the result of detailed research of rolling conditions of titanium containing steel within the composition range of the invention, it was found that even in Ti added steel, austenite grains which became coarse by heating will surely recrystallize into new fine grains by rolling at a temperature of more than ll00C. Also, we have found that if in rolling at less than 1 C a large reduction of more than 28% of draft percentage for one pass is effected, recrystallization become possible down to 980 C and the grains also become smaller. Moreover, even if the finishing temperature is as high as 880C, at which temperature the rolling will normally be completed, cold workability and low temperature toughness are remarkably improved. As the result of various further researches, we found that, in hot rolling process after heating of heating slab, by effecting a with more than one pass and with a draft percentage for one pass of more than 28% between ll00980 C, and making the finishing temperature of rolling lower than 880C and higher than 730C, titanium containing high tensile steel plate is obtained, which has superior cold workability and remarkably improved low temperature toughness. Thus, these are the specified rolling conditions of the high temperature finishing type in the present invention.

The characteristic of this high temperature finishing type resides in that said specific rolling of large reduction is effected more than one time between ll00980C, and of course the more times of said large reduction are effected, the more preferable. In this case, the rolling conditions over ll0OC are not specified, because when the rolling at higher than ll0OC is effected according to the conventional schedule (normally a total draft percentage of more than 50%) and in succession said specific large reduction is effected between ll00980C, the structure will recrystallize into finer austenite grains. Further, when the decrease of productivity is not a problem, steel having very superior cold workability and low temperature toughness can be obtained by omitting entirely the rolling at higher than 1 100C and by effecting a number of specific large reductions between I l00-980C.

In this high temperature finishing type, further rolling is effected after the specific large reduction rolling, and the rolling is finished below 880C and over 730C to give higher toughness and higher cold workability. This is done so that the recrystallized austenite grains are further rolled within a temperature range wherein recrystallization of austenite is impossible to make the ferrite grains fine after transformation, and thereby the low temperature toughness, particularly at the transition temperature, is improved as well as the superior cold workability are improved. Moreover, we recommend that total reduction below 980 C be more than 45% (preferably 50%) to realize the characteristics of the invention.

The reasons for limiting the rolling condition of high temperature finishing type are as above mentioned. However, said specific large reduction between ll00-980 C is not always possible in all rolling mills. In such rolling mills, the object of the invention may be attained by means of the low temperature finishing method.

In the low temperature finishing type of the invention, the rolling condition in the low temperature region is particularly important. Namely, since the finely recrystallized austenite grains cannot be obtained as in the high temperature finishing type, the structure after transformation is liable to become a ferrite structure of mixed grains or mixed with an upper bainite structure, and steel having such a structure is remarkably inferior in low temperature toughness and cold workability. To solve such a problem, it is necessary to effect a larger reduction in a lower temperature region than the high temperature finishing type in order to make the ferrite grains, after transformation, fine and uniform. As the result of various experiments, it has been clarified that the cold workability and the low temperature toughness are remarkably improved by making the total draft percentage below 980C more than 50% and by finishing the rolling at a temperature lower than 815C and higher than 730C, and this is the specified rolling conditions of the low temperature finishing method in the present invention. In addition, by making the total draft percentage below 850C more than 20%, the cold workability and the low temperature toughness can be improved further. The rolling conditions at higher than 980C are not specified, because the object of the invention can be attained so long as the rolling schedule at above 980C which has been effected conventionally (normally the total draft percentage at higher than 980C is effected with more than 60%) is performed and the rolling conditions in the low temperature region are satisfied according to the invention.

When the decrease of productivity is not a problem,

I the characteristics of the invention may be improved by minimizing the rolling at higher than 1 C and by increasing the total draft percentage below 1 100C. However, this is not a preferred method, because the productivity will be remarkably decreased and improvement of the quality of steel is not great.

The difference between the two types of rolling described above is that the former is available for higher rolling finishing temperatures and, therefore has high productivity and, as shown in some examples, the higher the rolling finishing temperature the higher strength can be obtained, so that it the high temperature finishing type is preferred.

But, in rolling mills which cannot carry out the high temperature finishing type, the object of the invention can be attained by'adopting the low temperature fin ishing type, even though the strength will somewhat decrease.

Even in the high temperature finishing type, by finishing the rolling below 8 1 5 and above 730C, which is the rolling finishing temperature of the low temperature finishing type, the low temperature toughness, particularly the transition temperature, is improved much more and has a superior transition temperature than the low temperature finishing type, so that in the invention, even in the high temperature finishing type, the rolling finishing temperature is below 880 and above 730C.

After the rolling is finished the steel may be aircooled in plate shape as is carried out in the ordinary thick plate process, or taken up in the coil shape. However, higher strength may be obtained when watercooled and coiled. In case the steel is coiled, the characteristics of the invention may be displayed to the utmost, by making the coiling temperature 500 to 680C and by making the cooling velocity from the finish of rolling to the coiling (hereinafter simply called the cooling velocity) between 3 and 40C/sec.

The invention will be further explained by the following examples.

Example 1. (High temperature finishing type) Table 2 shows various values of the characteristics of hot rolled steel plate having thickness of 8mm, which is obtained by rolling of the steel shown in Table 1 subject to the rolling condition shown in Table 2, i.e. mechanical properties of the hot rolled steel plate; minimum radius capable of bending 180 in the transverse direction (width of test piece 200mm) and hole expanding limit (initial hole diameter 20mm); vE-20 after 5% tensile strain representing low temperature toughness after cold working and vE-20 without pre-strain.

7 As is clear from Table 2, it is apparent that according to the high temperature finishing type (No. of rolling 2-5, 8-14) of the method of the invention, cold workability and low temperature toughness of steel plate are remarkably improved compared with the comparative method (No. of rolling l, 6, 7,

EXAMPLE 2 (High Temperature Finishing Type) Chemical compositions of the steel according to the invention and the comparative steel are shown in Table 3. These steels are molten and produced in converters, and after they are formed as slabs by way of ordinary process, they are rolled subject to the rolling condition shown in Table 4 into hot rolled steel plates of 8.0 mm thickness, of which various values of characteristics are shown in Table 4. And, coiling temperatures were 580-620C, and cooling velocity 7l 1C/sec.

Even in the case of steel having chemical compositions within the range of the invention, products having rolling numbers 16 and 19, which do not satisfy the rolling condition of the invention, are of the same grade in cold workability compared with rolled products from the comparative steel (No. of rolling 22,23), but low temperature toughness are inferior.

The rolled products having rolling numbers l7, 18, and 21 according to the high temperature finishing type of the invention have higher strength than the rolled products from the comparative steel. Moreover cold workability of the invention steels is remarkably more superior than those from the comparative steel and also their low temperature toughness is more than or equal to those from the comparative steel plate.

EXAMPLE 3 (High Temperature Finishing Type) Chemical compositions of the steel according to the invention and the comparative steel are shown in Table 5. These steels are molten and produced in converters, and after they are formed as slabs by way of ordinary process, they are rolled subject to the rolling condition shown in Table 6 into hot rolled steel plates of 6.0l0.0mm thickness, of which various values of characteristics are shown in Table 7.

As is clear from Table 7, the hot rolled steel plates produced according to the high temperature finishing type of the invention have higher strength and superior cold workability to any comparative steel as well as superior low temperature toughness more than or equal to the comparative steel.

Steel 2 of the invention made by adding Zr to steel 1 of the invention and steel 3 of the invention made by adding Nb and V to steel 1 are more improved than steel 1 of the invention in any of strength, cold workability and low temperature toughness, thus it is understood that the effect of the invention is still more displayed by the addition of Zr, Nb and V,

EXAMPLE 4 (Low Temperature Finishing Type) Table 8 shows various values of the characteristics of hot rolled steel plate having thickness of 8mm, which is obtained by rolling of the steel shown in Table 1 subject to the rolling condition shown in Table 8, i.e. mechanical properties of the hot rolled steel plate; minimum radius capable of bending 180 in the transverse direction I 8 and hole expanding limit representing cold workability of high tensile steel plate; and vE-2O after 5% tensile strain representing low temperature toughness after cold working and vE-20 without pre-strain.

As is clear from Table 8, according to the low temperature finishing type (No. of rolling 28, 29, 32, 33, 34) of the invention, the cold workability and the low temperature of the steel plate are remarkably improved compared with the comparative method (No. of rolling 25, 26, 27, 30, 31, 35, 36).

EXAMPLE 5 (Low Temperature Finishing Type) Steel according to the invention and comparative steel having chemical compositions shown in Table 3 are rolled subject to the rolling condition shown in Table 9 into hot rolled steel plate of 8.0mm thickness, of which various kinds of characteristic values are shown in Table 9. And, coiling temperature therefor are 580-620C and cooling velocity from the end of rolling to the coiling are 7-l 1C/sec.

Even though steel having chemical compositions within the range of the invention is used, the steel plate having rolling number 37, which does not satisfy the rolling condition of the invention, has equal cold workability to comparative steel plate (rolling number 22, 23), but the low temperature is inferior to the comparative steel plate.

The steel plate having rolling number 38 obtained according to the low temperature finishing type of the method of the invention has higher strength than com parative steel, nevertheless cold workability of the invention steel plate is remarkably superior and low temperature toughness thereof has characteristics of more than or equal to comparative steel plate.

EXAMPLE 6 (Low Temperature Finishing Type) Chemical compositions of the steel within the range of the invention and of the comparative steel are shown in Table 5. These steels are rolled subject to rolling condition shown in Table 10 into hot rolled steel plate having thickness of 6.0lO. Omm, of which various kinds of characteristic values are shown in Table l 1. As is clear from Table l l, the hot rolled steel plate formed according to the low temperature finishing type of the invention has higher strength and superior cold workability to the comparative steel as well as superior low temperature toughness more than or equal to the comparative steel Steel 2 of the invention made by adding Zr to steel 1 of the invention and steel 3 of the invention made by adding Nb and V to steel 1 are more improved than steel 1 of the invention in any of strength, cold workability and low temperature toughness, thus it isunderstood that the effect of the invention is still more displayed by the addition of Zr, Nb and V.

Having thus described the invention in details, the high tensile steel plate produced according to the invention has strength of more than Kg/mm having superior cold workability and low temperature toughness at the same time compared with usual high strength steel plates, and its carbon equivalent is low so that its weldability is very good. Among cold workabilities, drawability and stretchability are more than or equal to those of usual steel, so that the high tensile steel of the invention is not only adapted for automobile frame and the like but also best suited for making line-pipe because of good low temperature toughness as V 2.1 kg-m and V C according to and good weldability. Particularly, the present steel .118 No. 4, 5mm sub size specimen.

Table 1 Chemical Compositions of Examples 1 and 4 (wt%) C Si Mn 7 P S Ti Sol. N Ti/C Rest 0.10 0.33 1.27 0.012 0.006 0.12 0.031 0.0040 1.2 Fe and inevitable I elements Table 2 Rolling conditions and various kinds of characteristic values of Example 1 Rolling Condition Roll- Ro1l Heating Total Rolling Total Rolling ing ing temp.of draft between draft finishpattern No. slab percen- 1 C/ percening (C) tage 980C tage temp.

above below ("C) 1 100C 980C 1 920 (2 880 C 1300C 53% 29% 51.5% 840 (Times of 800 pass=1) 750 6 710 7 920 31% 880 D 1300C 28% 34% 51.5% 840 (Times of 810 pass=2) 780 30% 880 E 1300C 49% (Times of 48.0% 840 pass=1) 800 F 15 1300C 55% 25% 52.4% 860 (Times of pass=l) Note.

1) Encircled Rolling No. are belonging to the invention. 2) Water-cooling is effected from the end of rolling to 600C with velocity of 10C/sec. then furnace cooling from 600C.

Mechanical Properties Cold workability Low temperature No. 5) toughness (-115 N0. 4,sub-size 7.5mm)

Yield Tensile Elonga- Minimum rad- Hole point strength tion ius capable expanding vE-20 (Kg-m) (Kg/mm) (Kg/mm (70) of 180 limit bending in (d/do) without after transverse (do=20mm) pre- 5% direction strain .tensile (test piece strength width 200mm) 65.7 74.0 27 0.5 t 2.1 i 7.8 1.5 66.0 74.1 28.5 0 t 2.7 10.6 8.0 62.8 70.1 29 0 t 2.7 12.1 9.4 61.3 68.7 29 0 t 2.9 13.4 11.3 61.5 66.8 30 0 t 2.7 12.1 10.4 60.8 65.8 30 0.5 t 2.2 8.7 4.6 64.8 73.2 26.5 0.5 t 2.2 6.9 1.9 63.1 72.1 27 0 t 2.7 11.3 8.8 63.8 71.3 28 0 t 2.8 12.6 10.4 60.9 67.8 29 0 t 2.8 13.1 11.0 60.1 67.5 29 0 t 2.9 12.8 11.4 64.4 7.2.4 28 0 t 2.4 9.2 5.7 63.1 70.6 29 0 t 2.5 9.9 7.6 60.8 68.4 30 0 t 2.7 11.0 9.8 64.2 72.1 27 0.5 t 2.2 7.3 2.0

possesses an excellent low temperature toughness such Table 3 Chemical compositions of Examples 2 and 5 (wt C Si Mn P 01 Ti N Ti/C Nb Rest Steel of the 0.12 0.35 1.43 0.017 0.006 0.034 0.09 0.0032 0.75 Fe and inveninevition table com element para- 0.09 0.27 1.31 0.018 0.008 0.025 0.0048 0.04 tive steel Table 4 Rolling conditions and various kinds of characteristic values of Example 2 Rolling Conditions Kind Roll- Roll- Heating Total Each Total Rolling of ing ing temp. of draft draft draft finish- Steel pattern No. slab percenpercenpercening (C) tage tage tage temp.

above between below (C) Steel 16 30% 900 of G 1270C 56% 73% 850 35% the (Times of 800 ass=2) 1nvp en 19 900 H 1270C 69% 357r 73% 850 tion (Times of 800 pass=1 Com- 1 22 1270C 64% 25% 73% 850 para- 25-) live (Times of Steel pass=2) Times of pass=1 Note:

1 Encircles Rolling N0." are belonging to the invention 2) Coiling temperature 580 to 620C 3) Cooling velocity 7 to 11C/sec.

Mechanical Properties Cold workability Low temperature toughness(.1lS No. (.118 No. 4.sub-size 7.5mm) Remarks Yield Tensile Elonga- Minimum rad- Hole point strength tion ius capable expanding vE- (Kg-m) (Kglmm (Kg/mm (70) of 180 limit bending in (d/do) without after transverse (do=20mm) pre- 5% direction strain tensile test piece strain (width=200mm) 66.1 72.1 26 0.5 t 1.9 6.4 2.0 Comparative 61.9 68.4 28 0 t 2.8 11.3 10.2 example 59.7 66.8 29 0 t 2.9 12.1 10.5 The invention 64.2 71.9 27 0 5 t 1.9 5.8 1.9 Comparative 61.2 68.4 29 0 t 2.7 10.4 8.7 The invention 59.4 65.4 30 0 t 2.7 11.8 9.8 506 63.4 32 0 5 t 1.9 9.8 7.8 51.7 62.7 33 0 5 t 1.8 10.6 8.5

Table 5 Chemical Compositions of Examples 3 and 6 (wt Si Mn P S sol.Al N Ti Ti/C Others Rest Steel 1 of the invention 0.11 0.33 1.45 0.017 0.006 0.032 0.0044 0.18 1.64 Steel 2 0.11 0.33 1.45 0.017 0.006 0.032 0.0044 0.18 1.64 0.04% Zr Fe and Steel 3 0.11 0.33 1.45 0.017 0.006 0.032 0.0044 0.18 1.64 0.02% Nb. 0.01% V inevi- Stee14 0.10 0.37 1.48 0.018 0.005 0.035 0.0052 0.21 2.10 0.35% Cr. 0.0015% B table Steel 5 0.08 0.41 1.38 0.012 0.008 0.045 0.0047 0.10 1.25 0.24% Ni, 0.32% Cu elements Steel 6 0.10 0.26 1.10 0.011 0.008 0.018 0.0048 0.10 1.0 0.02% (La+Ce) Comparative 0.09 0.27 1.27 0.013 0.007 0.028 0.0051 0.04% Nb, 0.03% V steel g Table 6 Rolling conditions of Example 3 Rolling Heating Total draft Each draft 7: Total draft 7! Rolling coiling Cooling pattern temp. of slab above 1 100C between below 980C finishing temp. velocity 1100C/980C temp.

High temp. 1260-1320C More than 50% 35%- more than 840870 570-620C 3l3C/sec finishing type times of pass= Table 7 Various kinds of characteristic values of steel plates obtained by Example 3 Manufacturing Mechanical Cold Low temperature condition property workability toughness Rolling Thickness Yield Tensile Elonga- Minimum radius Hole vE-20 (kg-m) capable of 180 expandpattern of product point strength tion bending in transing limit without after 5% verse direction (d/do) pretensile (mm) (kg/mm) (kglmm (Test piece (do=20mm) strain strain width=200mm) Steel 1 of the High temp.

i invention finishing 6.0 63.8 71.4 27 t 2.4 6.8 4.9

type Steel 2 6.0 65.7 73.5 27 0 t 2.8 7.5 5.6 Steel 3 6.0 65.1 72.1 27 0 t 2.7 7.6 5.7 Steel 4 10.0 69.2 81.3 31 0 t no test 9.8 8.0 Steel 5 8.0 62.1 69.3 28 0 t 2.9 12.4 10.1 Steel 6 6.0 57.8 68.7 29 0 t 2.9 7.8 5.9 Comparative 6.0 52.3 64.5 30 0.5 t 1.6 6.7 47

Steel Test piece for impact test: Thickness of product (mm: .115 No. 4, Sub-size 5mm Thickness of product 8mm and 10mm: .118 No. 4. Sub-size 7.5mm

Table 8 Rolling conditions and various kinds of characteristic values of Example 4 Rolling; Condition 1) Encircles Rolling number are belonging to the invention 2) Watercooling is effected from the end of rolling to 600C with velocity of 10C/sec. then furnace cooling from 600C.

Mechanical Cold Low temperature Properties workability toughness (.118 No. 5) (HS No.

4, sub-size 7.5mm) Yield Tensile El0nga Minimum Hole expoint strength tion radius panding vE- (Kg-m) (Kg/mm' (Kg/mm) capable of limit 180C bend- (d/do) without after ing in (do=20mm) pre- 5% transverse strain tensile direction strain (test piece 7 g width=200rnm) 66.3 74.6 27.5 0.5 t 2.0 5.0 0.5 64.1 73.2 27 0.5 t 2.0 6.1 0.6 65.0 71.4 28 0 t 2.2 7.0 1.7 62.3 69.3 29 0 t 2.7 9.8 8.1 60.0 66.8 29 0 t 2.8 11.8 9.3 63.0 72.8 28 0.5 t 2.1 g 5.8 1.0

Rolling conditions and various kinds of characteristic values of Example 4 Rolling; Condition 61.5 68.6 29 0 t 2.7 11.3 9.3 60.8 67.9 29 0 t 2.7 12.4 9.9 60.7 65.5 30 0 t 2.6 9.9 8.7

Table 9 Rolling conditions and various kinds of characteristic values of Example 5 Kind Rolling Condition Roll Roll- Heating Total Each Total Rolling of ing ing temp. of draft draft draft finigh- Steel pattern No. slab percenpercen percening (C) tage tage tage temp.

above between below (C) 1 100C 1 100/ 980C Steel 1 37 1270C 64% 73% 850 of the 25% inven- (Times of 800 tion pass=2) Com- 1 22 1270C 64% 25%- 73% 850 para- 25% tive (Times of steel pass=2) (Times of pass=1) Mechanical Properties Cold workability Low temperature (115 No. 5) toughness (.118 No. 4. sub-size 7.5mm) Remarks Yield Tensile Elonga- Minimum Hole ex vE-20 (Kg-m) point strength tion radius cappanding (Kglmm (Kg/mm able of 180 iimit without after bending pre- 5% in trans (d/do) strain tensile verse dir- (do=20mm) strain ection (test piece width=200mm) 60.8 69.3 28 0.5 t 1.9 5.8 1.8 Comparative example 60.5 67.1 29 0 t 2.7 10.1 8.8 The invention Notes:

1) Encircles Rolling No." arc belonging to the invention 2) Coiling temperature 580 to 620C 3) Cooling velocity 7 to 11 C/sec.

Table 10 Rolling conditions of Example 6 Rolling Heating Total draft Each draft Total draft Rolling finish- Coiling Cooling patten temp. of above between below ing temp. temp. velocity slab 1 C 1 100C/980C 980C Low 1260- more 20% +22% more 780-810C 570- 3-13C/ temp. finish- 1320C than (Times of than 620C sec.

ing 50% pass=2) 50% type Table 1 1 Various kinds of characteristic values of steel plate obtained by Example 6. Manufacturing Mechanical property Low temperature condition (118 No. 5) Cold workability toughness Rolling Thickness Yield Tensile Elonga- Minimum radius Hole exvE-20 (kg-m) pattern of point strength tion capable of panding without after 5% product (kgImm (kg/mm (70) bending in translimit pre-strain tensile direction (test (d/do) strain piece width=200mm) (do=20mm) Steel 1 of Low temp. the invention finishing 6.0 61.2 69.4 27 O t 2.4 6.7 4.7

t 6 Steel 2 6.0 62.7 71.1 26 O t 2.7 6.8 5.1 Steel 3 6.0 63.1 71.4 27 0 t 2.8 7.2 5.3 Steel 4 10.0 67.8 80.6 30 0 t no test 9.7 713 Steel 5 8.0 61.2 68.3 28 O t 2.8 12.5 10.4 Steel 6 6.0 55.8 67.1 29 O t 2.9 7.7 5.7 Comparative Steel 1 6.0 53.1 64.7 29 0.5 t 1.6 6.6 4.5

Note: Test piece for impact test:

Thickness of product 6mm .llS No. 4. Sub-size 5mm Thickness of product 8mm and 10mm: .118 No. 4. Sub-size 7.5mm

What is claimed is:

1. In a method for manufacturing high tensile steel having superior cold workability and toughness wherein an aluminum killed steel slab is heated and hot rolled, the improvement wherein the steel contains 0.04 to 0.35% Ti, 0.005 to 0.15% soluble aluminum, less than 0.008% N, and Ti/C is less than 4, and the steel slab is heated to greater than l200C and the hot rolling is finished at a temperature between 730 and 880C and carried out with more than one pass at a temperature from 980 to llC and a reduction of more than 28%.

2. In a method for manufacturing high tensile steel having superior cold workability and toughness wherein an aluminum killed steel slab is heated and hot rolled, the improvement wherein the steel contains 0.04 to 0.35% Ti, 0.005 to 0.15% soluble aluminum, less than 0.008% N, And Ti/C is less than 4, and the steel slab is heated to greater than 1200C and the hot rolling is finished at a temperature between 730 and 815C and carried out at a temperature below 980C and a total reduction of more than 50%.

3. The method of claim 2 wherein-during the rolling, more than one pass is carried out at a temperature between 980 to 1100C and a reduction of more than 28% 4. The method of claim 1 wherein the steel contains 003 to 0.20% C, no more than 0.8% Si, 0.90 to 2.5% Mn, no more than 0.020% S, no more than 0.008% N, and the balance iron and unavoidable impurities.

5. The method of claim 1, wherein the steel contains one or more of the following: Zr 0.01 to 0.15%, La Ce=0.005 to 0.10%, Nb=0.01 to 0.10%, V =0.01 to 0.10%, B 0.0030% Ni 0.5%, Cr 5 0.5% and Cu 2 0.5%.

6. The method of claim 1 wherein after the hot rolling, the steel is coiled at a temperature in the range from 500 to 680C.

7. The method of claim 6 wherein after the hot rolling and prior to coiling, the steel is cooled at a rate from 3 to 40C /second.

8. The high tensile steel having superior cold workability and toughness produced by the process of claim 1.

9. The high tensile steel having superior cold workability and toughness produced by the process of claim 2. 

1. IN A METHOD FOR MANUFACTURING HIGH TENSILE STEEL HAVING SUPERIOR COLD WORKABILITY AND TOUGHENSS WHEREIN AN ALUMINUM KILLED STEEL SLAB IS HEATED AND HOT ROLLED, THE IMPROVEMENT WHEREIN THE STEEL CONTAINS 0.04 TO 0.35% TI, 0.005 TO 0.15% SOLUBLE ALUMINUM, LESS THAN 0.008% N, AND TI/C IS LESS THAN 4, AND THE STEEL SLAB IS HEATED TO GREATER THAN 1200*C AND THE HOT ROLLING IS FINISHED AT A TEMPERATUREBETWEEN 730* AND 880*C AND CARRIED OUT WITH MORE THAN ONE PASS AT A TEMPERATURE FROM 980* TO 11::*C AND A REDUCTION OF MORE THAN 28%.
 2. In a method for manufacturing high tensile steel having superior cold workability and toughness wherein an aluminum killed steel slab is heated and hot rolled, the improvement wherein the steel contains 0.04 to 0.35% Ti, 0.005 to 0.15% soluble aluminum, less than 0.008% N, And Ti/C is less than 4, and the steel slab is heated to greater than 1200*C and the hot rolling is finished at a temperature between 730* and 815*C and carried out at a temperature below 980*C and a total reduction of more than 50%.
 3. The method of claim 2 wherein during the rolling, more than one pass is carried out at a temperature between 980* to 1100*C and a reducTion of more than 28%
 4. The method of claim 1 wherein the steel contains 0.03 to 0.20% C, no more than 0.8% Si, 0.90 to 2.5% Mn, no more than 0.020% S, no more than 0.008% N, and the balance iron and unavoidable impurities.
 5. The method of claim 1, wherein the steel contains one or more of the following: Zr 0.01 to 0.15%, La + Ce 0.005 to 0.10%, Nb 0.01 to 0.10%, V 0.01 to 0.10%, B < or = 0.0030% Ni < or = 0.5%, Cr < or = 0.5% and Cu < or = 0.5%.
 6. The method of claim 1 wherein after the hot rolling, the steel is coiled at a temperature in the range from 500* to 680*C.
 7. The method of claim 6 wherein after the hot rolling and prior to coiling, the steel is cooled at a rate from 3* to 40*C /second.
 8. The high tensile steel having superior cold workability and toughness produced by the process of claim
 1. 9. The high tensile steel having superior cold workability and toughness produced by the process of claim
 2. 